The present invention relates generally to thrust bearing assemblies and, more particularly, to a starter apparatus which temporarily reduces the normal force between a thrust bearing and its associated runner during startup.
Thrust bearings are commonly utilized on rotating machinery such as large vertical-axis motors and hydrogenerators to prevent axial movement of the machinery's rotor. These thrust bearings generally have a static friction which is much higher than their dynamic friction. This is due to the relative dryness of the interface between the thrust bearing and its associated runner when in a static condition as compared to the wetted interface of an operating thrust bearing system, which benefits from a lower coefficient of friction because of the presence of a thin layer of oil located therebetween.
Due to this relative dryness, it is sometimes difficult to initiate the rotation of the bearing's associated rotor member when the bearing is under a high axial load. When the axial load on the thrust bearing is sufficiently high relative to the starting torque of the associated motor or generator, the static friction of the thrust bearing cannot be overcome and the associated machine will therefore not start.
The relationship between this normal force N and the resulting frictional force F is defined as EQU F=fN (1)
where f is the coefficient of friction between the thrust bearing and its associated bearing runner at the interface described above. As can be readily determined from the above equation, the frictional force F can be reduced by reducing either the coefficient of friction f or the normal force N.
This rotor starting problem is generally overcome with a hydraulic lift system which takes the approach of reducing the coefficient of friction f of the above equation. This type of lift system forces high-pressure oil into the interface between the bearing shoes and the shaft runner. The presence of oil between these adjacent surfaces reduces the coefficient of static friction therebetween and therefore reduces the minimum torque required to start the rotation of the associated rotor. Hydraulic lift systems typically require complicated labor-intensive installation and expensive parts, which include manifolds, orifices, check valves, hydraulic piping and fittings. The bearings themselves generally require special machining operations to be compatible with hydraulic lift systems. Also, the spatial requirements of hydraulic lift systems occasionally exceed the volume capacity of some bearing housings.
The present invention, rather than attempting to reduce the coefficient of friction between the adjacent surfaces of the bearing and shaft runner, temporarily reduces the normal force N between these two adjacent components. A thrust bearing assembly made in accordance with the present invention replaces the hydraulic lift system with means for imparting an axial force on the shaft, such as a shaft displacement cylinder, and an anti-friction bearing. High-pressure oil is supplied to a hydraulic cylinder which exerts an axial force on the rotor shaft in a direction which opposes and reduces the normal force N between the thrust bearing and its runner. Therefore, the pressure between the axial bearing and its adjacent shaft runner is relieved and, since the normal force N between the bearing and its runner is reduced, the static friction F is also reduced. This reduction of the friction force F between the bearing and its runner reduces the starting torque required for overcoming static friction and initiating rotation of the rotor shaft.
The present invention provides a thrust bearing starter assembly which is also much simpler to assemble and maintain than hydraulic lift systems and which utilizes parts which are less expensive and easier to install than comparative high-pressure oil systems as utilized to incorporate the hydraulic lift concept.